The present invention relates to an ink jet recording head of the type in which a pressure generating means applies pressure to ink within a pressure generating chamber, thereby shooting forth ink droplets through nozzle openings. More particularly, the invention relates to the structure of the pressure generating chamber.
One of the known pressure generating chambers communicates, at one end, with an ink tank and is provided with nozzle openings. The pressure generating chamber further includes a pressure generating means provided therein. A heater or a deformable means is used for the pressure generating means. The heater is used for evaporating part of the ink contained pressure generating chamber. The deformable means is realized by forming a diaphragm region within the pressure generating chamber. In this case, a mechanical drive means is provided for pushing the diaphragm region to cause a displacement thereof. With the displacement of the diaphragm region, ink is jetted out of the nozzle openings.
The pressure generating chamber with a piezoelectric vibrator of the longitudinal vibration mode which is brought into contact with the deformable means is known. The ink jet recording head using this type of pressure generating chamber is suitable for color printing, because it is free from the problem of changing ink quality by heat.
In the ink jet recording head operating such that the piezoelectric vibrator of the longitudinal vibration mode resiliently deforms the pressure generating chamber to cause ink to be jetted out of the chamber, the width of the array of nozzle openings of the pressure generating chamber can be remarkably reduced. Accordingly, a high resolution print is possible. When the size of the ink jet recording head is reduced under the constant resolution improvement pressure, the deformable means of the pressure generating chamber must be correspondingly thinned in order to reduce the rigidity of the deformable means. The thin deformable means is fragile.
In the ink jet recording head having the resolution of 300 DPI and having nozzle openings zig-zag arrayed in four lines, the length and the width of the pressure generating chamber are 1 to 2 mm and approximately 200 .mu.m, respectively. The deformable region of the vibrating plate constituting the deformable means is 1 to 3 .mu.m thick, and the region thereof to be displaced by the piezoelectric vibrator is several tens .mu.m. The fore end face of the piezoelectric vibrator for deforming the pressure generating chamber is approximately 500 .mu.m .times.200 .mu.m.
For this reason, the vibrating plate includes protruded portions, called islands, which are formed at the locations to be in contact with the piezoelectric vibrators, and longer than the vibrators. The island expands the displacement of the piezoelectric vibrator in the longitudinal direction of the vibrating plate, obtaining a good matching. With the use of the islands, stress concentrates at a part near to the boundary of the island and the flexible thin portion. Eventually, the vibrating plate is broken down by fatigue.
A protruded part, called an island part, is formed in the portion of the vibrating plate where it comes in contact with the piezoelectric vibrator. The island part is longer than the piezoelectric vibrator. A displacement of the piezoelectric vibrator is enlarged in the longitudinal direction. Since the vibrating plate is so shaped, stress is concentrated at the island part and in a boundary part where the deformable thin portion terminates. Accordingly, the vibrating plate is easy to be broken.
The fragility problem of the vibrating plate can be solved by making the thin portion of the vibrating plate as thick as possible. However the thin portion increases the pressure in generating ink droplets. In generating ink droplets, an abrupt increase of the pressure must be avoided; otherwise, misty ink is generated and the ink quality is deteriorated.